Direct current power supply for manual arc welding

ABSTRACT

A direct current power supply for manual arc welding having a steeply drooping, adjustable characteristic, comprising a polyphase transformer and a polyphase rectifier system including triggered rectifier means and a control system therefor providing both welding current and welding voltage feed-back. The welding current feed-back is substantially fixed. The welding voltage feed-back is adjustable for adjustment of the slope of the characteristic.

This is a continuation of application Ser. No. 352,446, filed Apr. 18,1973, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a direct current power supply for manual arcwelding a steeply drooping, adjustable characteristic. In this context,the expression "steeply drooping" should be understood to indicate thatthe short circuit current exceeds the arc current by not more than 50%.More particularly, the invention relates to a power supply of the typecomprising a polyphase transformer, a polyphase rectifier systemconnected to the secondary of said polyphase transformer, said rectifiersystem comprising a set of triggered rectifier valves connected one inseries with each secondary phase circuit of the polyphase transformer, apair of welding current conductors connected to the output of saidpolyphase rectifier system, means for controlling the firing angle ofthe triggered rectifier system in response to a direct current controlsignal, and signal generating means for producing said direct currentcontrol signal comprising

Means for producing an adjustable direct current reference signal,

Means including a welding current sensing means for producing a weldingcurrent feed-back direct current signal, and,

Summing means for combining said reference signal and said weldingcurrent feed-back signal into a compound signal constituting the controlsignal aforementioned, said feedback signal being supplied to saidsumming means in opposing relationship to said reference signal.

In a welding power supply of this kind described in the U.S. Pat. No.3,530,359, the adjustable D.C. reference signal is obtained by thecombining of two component signals, both of which are individuallyadjustable. More particularly, one adjusting member is provided for thefirst component, while two adjusting members independent of each otherand of said first-mentioned adjusting member are provided for the secondcomponent. One of said two last-mentioned adjusting members also servesto control the slope of the characteristic of the power supply throughadjustment of the factor of proportionality between the welding currentfeed-back signal and the welding current. Consequently, the properadjustment of the apparatus involves the manipulation of threeindependent controls, which is apt to confuse the welder. Also, thewelding current delivered by a power supply of this kind varies stronglywith the line voltage. The disturbing action of line voltagefluctuations increases with the forward slope of the characteristic. Tominimize the disturbing action of line voltage fluctuations the slopecontrol has to be set at as steep a slope as possible, that is, a slopeapproaching the vertical as nearly as possible. On the other hand, it isa well-known fact that, in welding with coated electrodes, the shortcircuit current must exceed the arc current by not too small an amountin order to ensure an orderly metal transfer from the electrode to theworkpiece and to prevent "stubbing" and other undesirable effects. Withthe control system referred to it is, therefore, not possible to combinegood welding characteristics with the desired insensitivity to linevoltage fluctuations.

SUMMARY OF THE INVENTION

The present invention provides an improved arc welding power supply ofthe type above specified which does not exhibit the disadvantages of thecontrol system previously employed. In particular, the inventionprovides a control system in which the action of line voltagefluctuations upon the welding current can be substantially suppressed orkept within acceptable limits and in which the slope of the power supplycharacteristic can be adjusted as desired substantially without anyresulting increase of the sensitivity to line voltage fluctuations.

According to one aspect of the invention, the signal generating meansfor producing the direct current control signal supplied to the meansfor controlling the firing angle the triggered rectifier systemcomprises means for producing an adjustable direct current referencesignal.

means including a welding current sensing means for producing a weldingcurrent feed-back direct current signal bearing a substantially fixedproportion to the welding current,

means including a welding voltage sensing means for producing a weldingvoltage feed-back direct current signal proportional to the weldingvoltage,

summing means for combining said reference signal, said welding currentfeed-back signal and said welding voltage feed-back signal into acompound signal constituting the control signal aforementioned, both ofsaid feed-back signals being supplied to said summing means in opposingrelationship to said reference signal,

said means for producing a welding voltage feed-back signal includingmeans for adjustment of the slope of the power supply characteristicthrough adjustment of the factor of proportionality between said voltagefeed-back signal and the welding voltage.

The invention also includes a control system for a welding power supplyof the type stated in which the required insensitivity to line voltagefluctuations is secured by the use of a sufficiently large weldingcurrent feed-back factor and in which adjustable feed-back of thewelding voltage is provided in order to allow the required adjustment ofthe slope of the welding supply characteristic. The term "weldingcurrent feed-back factor" means the quantity ΔV/ΔI, in which ΔI is themagnitude of a change of the current through the welding current sensingmeans and ΔV is the change of the output voltage of the rectifier systemresulting from said change of current. The determination of saidquantity must be carried out under the following conditions: The controlloop must be open (no feed-back); the transformer must be connected tothe normal line voltage; the rectifier must be adjusted to provide anoutput voltage within the arc voltage range. According to the invention,the welding current feed-back factor thus determined should not be below1.5 volts/ampere, preferably not below 2.5 volts/ampere. Feed-backfactors exceeding 5 volts/ampere are best avoided as they may causeinstability of the control system.

Further advantages and benefits of the present invention will becomeapparent from the following description of embodiments of the invention,taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a first embodiment of a welding powersupply according to the invention.

FIG. 1a shows a modified form of part of the circuit of FIG. 1.

FIG. 2 is a voltage-current diagram illustrating the mode of working ofthe circuit of FIG. 1.

FIG. 3 is a circuit diagram of a second embodiment of a welding powersupply according to the invention.

FIG. 4 is a voltage-current diagram illustrating the mode of working ofthe circuit of FIG. 3.

FIG. 5 is a detailed circuit diagram of a preferred embodiment of theinvention.

FIGS. 6 and 6a are voltage-current diagrams illustrating the mode ofworking of the circuit of FIG. 5.

FIG. 7 is a circuit digram of one of the triggering circuits formingpart of the apparatus of FIG. 5, and,

FIGS. 8a to 8e are voltage-time diagrams illustrating the mode ofworking of the triggering circuit of FIG. 7.

DESCRIPTION OF THE EMBODIMENTS

The welding power supply of FIG. 1 comprises a three-phase transformer 1having input terminals R, S, T, connected to a three-phase power line.The secondary of said transformer is connected to a rectifier 2 providedwith triggered rectifier valves. The transformer 1 has a low magneticleakage and may be a power transformer of the usual type in which thewindings are carried by a common, three-legged core. A welding circuitconnected to the output of the rectifier contains a series inductor 3and a pole-changing switch 4 by which any desired polarity of thewelding electrode 5 with respect to the workpiece 6 can be obtained. Afreewheel diode 7 is connected between the end of the inductor connectedto the rectifier and the other welding current conductor. The rectifier2 is controlled by a triggering unit 8 emitting triggering impulsessynchronized with the line voltage, the phase angles of said triggeringimpulses being controlled in response to a control signal supplied tothe triggering unit by amplifier means 9. The input lead of saidamplifier means 9 is connected to a summing circuit comprising threeresistors 10, 11, 12. The resistor 12 is connected to an adjustable tap13a of a potentiometer 13 connected to a source of constant voltage 14.Thus, the resistor 12 conducts a constant current the magnitude of whichis adjustable by means of the tap 13a. The resistor 10 is connected to awelding current sensing means comprising a resistor 15 of smallmagnitude connected in series with one of the welding currentconductors. The resistor 11 is connected to an adjustable tap 16a of apotentiometer 16 connected between the welding current conductors. Thecurrents flowing through resistors 10, 11 are proportional to thewelding current and the welding voltage, respectively. The points marked"P" in the diagram are galvanically connected with each other byconductor means not shown and define the zero potential of the controlsignal generating circuit. The input signal supplied by the summingcircuit 10-12 to the amplifier means 9 is proportional to the algebraicsum of the currents flowing through the resistors. The polarities arearranged so as to make the currents in resistors 10, 11 oppose thecurrent in the resistor 12. Consequently, the input signal of theamplifier means 9 as well as the output voltage of said amplifier meansare equal to the difference between a constant term and the sum of twoterms varying respectively with the welding current and the weldingvoltage. The resulting control signal is supplied to the triggering unit8 and determines the phase angle of the triggering pulses supplied bysaid triggering units to the rectifier 2. The described feed-back of thewelding current and the welding voltage impart to the rectifier asteeply drooping characteristic determined by the setting of thepotentiometers 13 and 16. More particularly, the setting of thepotentiometer 13 determines the magnitude of the short circuit current,while the setting of the tap 16a of the potentiometer 16 determines thedroop, or angle of inclination, of the characteristic.

According to the invention, the welding current feed-back factor shouldamount to at least 1.5 volts/ampere in order to prevent variations ofthe line voltage supplied to terminals R, S, T from causinginconveniently large variations of the welding current. A determinationof the welding current feed-back factor can be carried out in thefollowing way. The connections between the points x, x¹ and between thepoints y and y¹ are broken. An independent source of D.C. voltage isconnected between the points x and y, its positive terminal beingconnected to the point x. An independent source of direct current isconnected between the points y and z, its positive terminal beingconnected to the point z. The output voltages of one or both of saidindependent D.C. sources are adjusted so as to make the rectifier 2supply an output voltage within the range of normal welding voltages.The D.C. source connected between points y and z is now adjusted so asto alter the current flowing through the resistor 15 by a certainamounts ΔI, for instance 1 ampere. The resulting change ΔV (in volts) ofthe output voltage of the rectifier 2 is determined. The rate ΔV / ΔI isthe current feed-back factor.

Theoretically, the determination of the current feed-back factor shouldbe carried out with the rectifier connected to a load resistor adjustedso as to make the rectifier deliver a current of the same magnitude asthe arc current corresponding to the output voltage of the rectifier.However, the error caused by carrying out the determination underno-load conditions, as above described, is negligible.

It will be realized that the welding current feed-back factor is afunction of the resistance of the resistor 15, the resistance of theresistor 10, the amplification of the amplifier means 9 and theamplification of the system provided by the rectifier 2 itself and itstriggering unit 8. The amplification of said lastmentioned system andusually also the resistance of the resistor 15 being constant factors,the resistance of the resistor 10 and/or the amplification of theamplifier means 9 have to be adjusted so as to provide the desiredwelding current feed-back factor.

In addition to the tap 16a, the potentiometer 16 has an adjustable tap16b connected to the tap 16a by a capacitor 17. The tap 16b is connectedto a point of the potentiometer 16 having a higher potential than thetap 16a. The capacitor 17 transmits no current as long as the weldingvoltage is constant and therefore does not modify the staticcharacteristic. When there occurs a change of the voltage between thewelding current conductors, a charging or discharging current flowsthrough the capacitor 17, resulting in a transient increase of thechange of the current through the resistor 11. For the duration of saidcharging or discharging current through the capacitor 17, the controlcircuit therefore adjusts the characteristic to a slope less steep thanthe static one. When a short circuit occurs between the electrode andthe workpiece, the control circuit thus provides a transient shortcircuit current exceeding the one determined by the staticcharacteristic.

On termination of the short circuit condition, the circuit describedtransiently adjusts the power supply characteristic to provide an arccurrent lower than the one determined by the static characteristic. Themodified circuit shown in FIG. 1a avoids this effect, which is notdesirable and may be harmful. In the circuit of FIG. 1a, the capacitor17 is connected to the tap 16a through a rectifier valve 18 poled so asto allow discharge only of the capacitor. To allow charging of thecapacitor, a second rectifier valve 19 connects the capacitor to thezero potential end of the potentiometer 16. By this means the chargingcurrent through the capacitor 17 occurring on increase of the voltagebetween the welding current conductor is prevented from causing anychange of the current through the resistor 11.

FIG. 2 illustrates the action of the circuit. The line 20 is thecharacteristic of the power supply under the assumption that therectifier valves are conducting during the complete A.C. period. Theline 21 is a normal or schematized characteristic of the welding arc.The lines 22 and 23 are two static characteristics of the power supplyproviding the short circuit current I₀₁ and I₀₂ (corresponding to twodifferent settings of the potentiometer tap 13a). The characteristicsare represented as having different slopes (corresponding to twodifferent settings of the tap 16a). The dotted lines illustrate, thetransient reduction of the slope angle and resulting transient increaseof the short circuit current produced by the action of the capacitor 17in FIG. 1 or 1a.

The embodiment of the invention schematically shown in FIG. 3 comprisesa three-phase transformer 26 the secondary of which is connected to amain rectifier 27 and an auxiliary rectifier 28. The main rectifier isprovided with triggered rectifier valves to allow the control of theoutput current. The auxiliary rectifier 28 comprises plain(non-controlled) rectifier valves only. Both of the rectifiers arearranged to feed one and the same welding circuit. The auxiliaryrectifier is connected in series with a resistor 29 limiting the shortcircuit current of the auxiliary rectifier to a low value. The weldingcircuit contains a series inductor 30. A free-wheel diode 31 isconnected between the conductor connecting the inductor with a powersupply and the other welding current conductor. The rectifier 27 iscontrolled by a triggering unit 32 to which a control signal is suppliedby an amplifier device 33. The input end of the amplifier device 33 isconnected to a summing circuit comprising four resistors 34, 35, 36, 37.The resistor 37 is connected to a welding current sensing means 38provided in one of the welding current conductors, resulting in a flowof current through the resistor 37 proportional to the welding current.The resistor 36 is connected to an adjustable tap 39 of a potentiometer40 connected to a source of constant D.C. voltage 41. The resistor 35 isconnected to an adjustable tap 42 of a potentiometer 43 supplied by acomparing circuit 44 with a voltage varying with the welding voltage.The welding voltage between the welding current conductors and aconstant voltage supplied by a potentiometer 45 connected to a source ofconstant voltage 46 are fed separately into the comparing circuit 44,which is arranged to supply to a conductor 47 a voltage proportional tothe difference between the welding voltage and a constant voltage V_(r),said voltage V_(r) being equal to or bearing a predetermined proportionto the voltage supplied by the potentiometer 45. The output voltage ofthe conductor 47 is supplied to a potentiometer 48 the tap 49 of whichis connected to the potentiometer 43 above referred to. A capacitor 50connected between the conductor 47 and the tap 49 of the potentiometer48 has a function identical with the one of the capacitor 17 of thecircuit of FIG. 1.

The input voltage or control voltage provided by the summing circuit 35,36, 37 contains the following (static) components:

a. A constant component adjustable by means of the potentiometer 40,

b. a constant component adjustable by means of the potentiometers 48 and43, said component acting in the same sense as the component a,

c. a component varying with the welding voltage and opposed to thecomponent a, the factor of proportionality between said component andthe welding voltage being adjusted by means of the potentiometers 48 and43,

d. a component opposed to the component a and varying with the weldingcurrent (supplied by the current sensing means 38).

The sum of the adjustable components a and b constitutes the constantreference component of the control signal and corresponds to thereference signal supplied in the circuit of FIG. 1 by the potentiometer13 and the resistor 12. The control system operates in the same way asthe circuit of FIG. 1. The system according to FIG. 3 provides, however,a certain practical advantage over the system of FIG. 1. In the systemof FIG. 1, the setting of the reference component of the control signalis carried out by means of the potentiometer tap 13a. Displacing of saidtap results in a displacement of the characteristic parallelly toitself. The slope of the characteristic is adjusted by means of thepotentiometer tap 16a. A change of the position of said tap produces arotation of the characteristic about the intersection of thecharacteristic and the current axis. This angular displacement of thecharacteristic results in a displacement of the operation point (thatis, the intersection of the characteristic of the power supply and thearc characteristic). For instance, adjustment of the characteristictowards lower slope angles displaces the point of operation towardslower currents. As this displacement is not always negligible, theposition of the tap 13a cannot be relied on to indicate the precisecurrent setting of the power supply. In the circuit of FIG. 3,displacement of the tap 39 of the potentiometer 40 shifts the powersupply characteristic parallelly to itself, while displacement of thetap 49 of the potentiometer 48 rotates the characteristic about itsintersection S (FIG. 4) with the line V = V_(r) parallel with thecurrent axis, V_(r) being equal to (or proportional to) the constantvoltage supplied in opposition to the welding voltage in the comparingcircuit 44.

FIG. 4 shows, in addition to the characteristic 54 of the rectifier whenfully conducting, a number of controlled characteristics 55-60. Thedotted line indicates the constant reference voltage V_(r). It followsfrom the above explanation that the change of arc current attending achange of the slope of the characteristic at a given setting of thepotentiometer tap 39 (resulting in a rotation of the characteristicabout the point S) is proportional to the distance between the point Sand the arc characteristic 61. V_(r) may for instance be about equal toor only a little larger than the largest arc voltage which can occur. Inthis case, the changes of the arc current I_(a) caused by changes of theslope setting are small enough to allow the provision of a realisticcurrent graduation on the potentiometer 40. Still better precision canbe obtained by using a reference voltage V_(r) equal to an arc voltageat the middle of the arc characteristic.

It has been established that a certain relation between the magnitude ofthe arc current and the slope of the characteristic is required foroptimum welding properties. More particularly, the relative increase ofthe static short circuit current over the arc current, or 100. I_(o) -I_(a) /I_(a) should be approximately constant at least within a mediumrange of currents. At high welding currents, a somewhat smaller relativeincrease of the short circuit current may be allowable and suitable. Thecharacteristics of FIG. 4 illustrate these maxims. The characteristics55 to 58 have slope angles which decrease with increasing currentsettings, while the characteristics 59, 60 at the upper end of the arccurrent range have the same slope as the characteristic 58. In somecases it may even be advisable to provide for a decrease of the slopewith increasing currents at the upper end of the current range, in orderto avoid overload of the rectifier valves. At the lower end of thecurrent range, a higher relative increase of the short circuit currentthan the one used for medium current settings is allowable and sometimesuseful. This is particularly true for arc currents lower than 50amperes. The relative increase of the short circuit current must,however, never exceed 50% of the arc current.

Preferably means linking the setting of the potentiometers 40 and 43 areprovided in order to establish a predetermined relation between thewelding current setting and the slope of the characteristic. In FIG. 3,said link means are represented as a common mechanical operating member51 for the potentiometer taps 39 and 42, the characteristics of thepotentiometers being assumed to be adapted to each other in such a wayas to provide the desired relation. Said linking means also ensures thatthere will be a definite relation between the current and the setting ofthe adjustable tap 39 (and the operating member 51) irrespectively ofthe choice of the constant voltage V_(r).

The transient control signal component supplied to the amplifier devicethrough the capacitor 50 is reduced by the potentiometer 43 in the sameproportion as the static component. At the lower end of the currentrange, the transient control signal thus produced and the resultingtransient current increase on short circuit is too low. To provide acontributory transient control voltage, a resistor 34 forming part ofthe summing circuit at the input of the amplifier device 33 is connectedto the output conductor 47 of the comparing circuit 44 through a circuitcomprising a series capacitor 52 and a resistor 53.

At low welding currents, the current supplied by the main rectifier iscomposed of comparatively brief pulses separated by currentlessintervals. The low current supplied by the auxiliary rectifier 28 inseries with the resistor 29 bridges said currentless intervals,resulting in an improved arc stability at low currents. In theembodiment of FIG. 3, the auxiliary rectifier is connected to the sametransformer winding as the main rectifier 27. It is, however, equallypossible to connect the auxiliary rectifier to a separate winding of thetransformer 26 or to a separate transformer, preferably with a highersecondary voltage than the winding feeding the main rectifier.

When the welding current feed-back factor of the control system of FIG.3 is to be determined, it is necessary either to disconnect theauxiliary rectifier 28 or to provide a load for the power supply unit.

The welding power supply of FIG. 5 has a main transformer comprising aprimary 65 and a secondary 66. An auxiliary winding 67 carries nowelding power but only supplies a three-phase voltage required for thecontrol system. The secondary is connected to a main rectifier composedof three diodes 68 and three thyristors 69 and is also connected tothree diodes 70 constituting together with the diodes 68 an auxiliaryrectifier. One output conductor 77 of the rectifier is connected to thewelding electrode 71. The other output conductor 76 is connected to thework 74 through a resistor 72 constituting the current sensing means ofthe control system and a welding inductor 73. The auxiliary rectifier isconnected to the welding circuit in series with a current limitingresistor 75. A free-wheel diode 78 is connected between the weldingcurrent conductors 76, 77.

The three thyristors 69 are each controlled by a triggering unit 79, 80,81, each of said triggering units being connected to the control lead ofa thyristor 69 through a triggering pulse conductor 82, 83, 84 (a partonly of which is represented in the drawing). The circuit diagram ofsaid triggering units is shown in FIG. 7.

The auxiliary operating voltages required for the control system aresupplied by a single-phase transformer 87 the secondary 86 of which isprovided with a centre tap 85. The secondary voltage is rectifier by afull-wave rectifier 88. The rectified voltage is smoothed by a filtercomposed of capacitors 89 and resistors 90 and is stabilized by means ofa pair of Zener diodes 91 connected in series between the positiveoperating voltage conductor 93 and the negative operating voltageconductor 94. The junction of the Zener diode is connected to the centretap 85 and to a zero potential conductor 92.

The positive terminal of the main rectifier is connected to the zeropotential conductor 92 through a conductor 95. The voltage of the otherterminal of the main rectifier, which is negative with reference to thezero potential conductor 92, is supplied to a conductor 97 through asmoothing R C -filter 96. The conductor 97 is connected to one end of aresistor 98 the other end of which is connected to the input of anoperational amplifier 99. Another resistor 100 connects said input tothe constant positive voltage of the operating voltage conductor 93. Thecontrol voltage supplied to the operational amplifier is, therefore,proportional to a current equal to the quotient of the constant voltageof the conductor 93 divided by the resistance of the resistor 100,reduced by the quotient of the welding voltage of the conductor 97divided by the resistance of the resistor 98. The operational amplifier99 is provided with a feed-back circuit comprising a resistor 101 inparallel with a diode 102. At zero input voltage, the operationalamplifier has zero output voltage. At positive input voltages, theoperational amplifier produces a negative output voltage proportional tothe input voltage. At negative input voltages (that is, if the weldingvoltage derived component of the input voltage is larger than theconstant component), the operational amplifier produces zero outputvoltage, the diode 102 preventing the appearance of a positive outputvoltage. If K denotes a proportionality constant, V_(s) the weldingvoltage and V_(r) a constant voltage, the operational amplifier producesan output voltage V_(out) = K(V_(s) - V_(r)), provided V_(s) is smallerthan V_(r), and an output voltage V_(out) = O, when V_(s) is equal to orexceeds V_(r). Said output voltage is supplied to a potentiometer 103connected between the output lead of the operational amplifier 99 andthe zero potential conductor 92. The tap 104 of said potentiometer isconnected to a second potentiometer connected between said tap and thezero potential conductor 92, said second potentiometer consisting of aresistor 105 in series with a resistor 106. The resistor 105 is a lightsensitive resistor arranged to be actuated by the light radiation of aphoto diode 105a. An increase of the radiation actuating the resistor105 produces a decrease of the resistance of said resistor. Thepotentiometers 103 and 105-106 correspond to the potentiometers 48 and43, respectively, in FIG. 3. The junction of the resistors 105 and 106is connected to one end of a resistor 107, the other end of which isconnected to an input lead 108 of an amplifier device 109 correspondingto the amplifier device 33 of FIG. 3.

A capacitor 110 having a function identical with the one of thecapacitor 50 of FIG. 3 is connected between the output lead of theoperational amplifier 99 and the potentiometer tap 104. Moreover, acapacitor 111 in series with a resistor 112 is connected between theoutput lead of the operational amplifier 99 and the zero potentialconductor 92, the junction of the elements 111 and 112 being connectedto the input lead 108 of the amplifier device 109 through a resistor113. The function of the circuit comprising said elements is identicalwith the function of the corresponding circuit 52, 53, 54 of FIG. 3.

A potentiometer 114 is connected between the zero potential conductor 92and the negative operating voltage conductor 94. The adjustable negativevoltage provided by the tap 115 is supplied to the photo diode 105a inseries with a resistor 116 and is also supplied to one end of a resistor117 the other end of which is connected to the input lead 108 of theamplifier device 109.

The small voltage drop caused by the flow of the welding current throughthe resistor 72 is smoothed by a filter comprising a resistor 178 and acapacitor 179 and connected through a series resistor 118 to the inputlead of an operational amplifier 120 provided with a feed-back resistor119. The output voltage of said amplifier, which is positive withrespect to the zero potential conductor 92 and proportional to thewelding current is supplied to one end of a resistor 149 the other endof which is connected to the input lead 108 of the amplifier device 109.

Accordingly, the control voltage supplied to the input lead 108 of theamplifier device 109 comprises the following static components:

a. A constant, negative component adjustable by means of thepotentiometer 114,

b. a component supplied by the operational amplifier 99 throughpotentiometers 103 and 105-106, said component being zero as long as thewelding voltage exceeds a predetermined value V_(r) and assuming anegative value proportional to the difference V_(r) - V_(s) for weldingvoltages V_(s) lower than V_(r),

c. a positive component supplied by the operational amplifier 120, saidpositive component being proportional to the welding current.

The amplifier device 109 comprises an operational amplifier 121 and afeed-back circuit therefor comprising three branches connected inparallel, the first branch containing a resistor 122, the second branchcontaining a resistor 123 in series with a capacitor 124, and the thirdbranch containing a diode 125 poled so as to block the appearance ofnegative output voltages. The operational amplifier 121 as well as theother operational amplifiers comprised in the circuit produces an outputvoltage the polarity of which is opposite to the polarity of the inputvoltage.

The output signal of the amplifier device 109 is supplied through aconductor 126 to each of the three triggering units 79, 80, 81. Theauxiliary voltages required for the operation of the triggering unitsare supplied to the triggering units by conductors 127, 128 and 129connected to the operation voltage conductors 93, 94 and the zeropotential conductor 92, respectively. Moreover, each of the triggeringunits is supplied with a pulsating voltage produced by a rectifierdevice connected to the auxiliary winding 67 of the main transformer,said rectifier device comprising three branches connected in parallel toeach other, each of said branches comprising a pair of rectifier valves130, 131 in series with a resistor 132. The terminals of the auxiliarywinding 67 are each connected to the junction between the two rectifiervalves in one of the branches. The junction of the three resistors 132is connected to the negative operating voltage conductor 94. Thepositive voltage appearing across each of the three resistors issupplied through conductor 133, 134, 135, respectively, to onetriggering unit 79, 80, 81, respectively. The circuit arrangements andthe function of the triggering unit are described in detail below withreference to FIGS. 7 and 8. It is sufficient to mention here that theangular positive of the triggering pulses produced by the triggeringunits varies in response to the magnitude of the control signal suppliedthrough the conductor 126 in such a way that the thyristors remain inthe non-conducting state as long as the control voltage is zero, andthat a control voltage above zero maintains the thyristors in theconductive state during a period increasing with the magnitude of thecontrol voltage.

FIG. 6 shows a group of characteristics corresponding to differentsettings of the potentiometer 114. The reference numerals correspond tothose employed in FIG. 4. Each of the control characteristics comprisesa sloping portion, for instance 56, corresponding to the slopingcharacteristic of FIG. 4, but extending up to the voltage V_(r) only,and a vertical portion of the voltage range between V_(r) and themaximum voltage determined by the characteristic 54 of the power supplywhen the thyristors are fully conducting. Said vertical portion is dueto the function of the amplifier 99 described above, its output voltageremaining at zero for all welding voltages above V_(r), whereby in thisvoltage range the current feed-back only is active. The voltage V_(r)should preferably have a value within the range of 40 to 50 volts.

The slope of the characteristic should have a certain relation to thecurrent setting; as already explained with reference to FIG. 4, thesteepness of the characteristic should be reduced with increased currentsettings up to a certain current setting, but may be maintainedapproximately constant in the upper current range (the characteristics58-60). In the apparatus of FIG. 5, said relation between the steepnessof the characteristic and the current setting has been obtained bysuitable adjustment of the characteristic of the control unitconstituted by the photo diode 105a and the light sensitive resistor105. More particularly, the properties of said control unit are suchthat, up to a certain limit, an increase of the voltage supplied by thepotentiometer 114 causes a reduction of the resistance of the resistor105 which, however, remains constant when the voltage is increased abovesaid limit.

With reference to FIG. 4 is was stated that the voltage V_(r) may bechosen within the arc voltage range. In the system according to FIG. 5the line V = V_(r), however, should not intersect the arccharacteristic, as the load point should be within the sloping portionof the characteristic. It is possible, however, to make the differencebetween the arc voltage and the reference voltage V_(r) as small asdesired by providing a reference voltage V_(r) which varies with thecurrent setting.

FIG. 6a illustrates this possibility, which can be realized for instanceby using a variable resistor 100 and provide a link between itsadjusting member and the adjusting member of the potentiometer 114 suchthat the resistance of the resistor 100 is reduced at increasingpotentiometer voltages.

In the previous description of the action of the amplifier device 109 itwas assumed that its second input lead 136 has the zero potentialdefined by the zero potential conductor 92. The apparatus according toFIG. 5 comprises, however, a special circuit supplying a negative biasvoltage to the input lead 136 as long as no current flows in the weldingcircuit. When thus biased, the amplifier device 109 produces no outputvoltage, so that the thyristors of the welding rectifier are entirelynon-conductive. The purpose of this arrangement is to prevent a strongcurrent surge when the welding operation is initiated.

The special circuit referred to comprises two transistors 137, 138. Theemitters 139, 140 of the transistors are connected to the zero potentialconductor 92, while the collectors 141, 142 are each connected to thenegative operation voltage conductor 94 through a resistor 143, 144,respectively. The collector 141 of the transistor 137 is connected withthe base of the other transistor 138 through a resistor 145. The base ofthe transistor 137 is connected to the junction P1 of two resistors 146,147, one 146 of which is connected to the positive operating voltageconductor 93, while the other resistor 147 is connected to the conductor97 carrying the smoothed negative output voltage of the weldingrectifier. The resistors 146, 147 are so adjusted that the point P1remains at negative polarity with respect to the zero potentialconductor 92 as long as the voltage of the conductor 97 is high (nearthe no-load value), but assumes positive polarity with respect to thezero potential conductor when the output voltage of the weldingrectifier drops below a predetermined value. The collector 142 of thetransistor 138 is connected to the input lead 136 of the amplifier 121through a resistor 148. The input lead 136 is also connected to the zeropotential conductor 92 through a resistor 150.

The action of the circuit described is as follows. As long as theconductor 97 is at the no-load voltage, the point P1 is at negativepotential, so that the transistor 137 is conducting while the transistor138 is non-conducting. Under these conditions the input lead 136receives a negative voltage, more particularly, the fraction of thenegative operating voltage determined by the potentiometer arrangement144, 148, 150. The output voltage of the amplifier device 109 is zero,and the thyristors are non-conductive. When the output voltage of therectifier drops sufficiently to render the voltage at the point P1positive, the transistor 137 is rendered non-conducting, while thetransistor 138 is rendered conducting and connects the input lead 136 ofthe amplifier 121 to the zero potential conductor 92 through theresistor 148.

It will be clear from the above description that the auxiliary rectifierconstituted by the diodes 70 together with the diodes 68 of the powersupply unit according to FIG. 5 has the additional function ofmaintaining a no-load voltage and supplying the initiating currentrequired to operate the special circuit described. FIG. 7 shows thecircuit arrangements of the triggering unit 79 of FIG. 5. The circuitincludes the conductor 127 connected to the positive operating voltageconductor 93, the conductor 129 connected to the zero potentialconductor 92, the conductor 128 connected to the negative operatingvoltage conductor 94, the conductor 133 supplying the synchronizedvoltage pulses appearing across one of the resistors 132, the conductor126 supplying the control signal produced by the amplifier 109, and theconductor 82 connected to the firing lead in one of the thyristors ofthe main rectifier.

The direct current pulses transmitted by the conductor 133 are suppliedto a potentiometer composed of two resistors 151, 152, the junction ofwhich is connected to the base of a transistor 153, the emitter of whichis connected to the negative operating voltage through the conductor128. The collector of the transistor 153 is connected to the conductor127 carrying the positive operating voltage through an RC-devicecomprising a capacitor 154, the charging resistors 155, 156, 157, adischarging resistor 158 and two diodes 159, 160. The resistor 155 isadjusted to provide a desired charging time constant. The junction P2between the resistor 156, the capacitor 154 and the diode 159 isconnected to the base of a transistor 162 and is also connected to thezero potential conductor 129 through a diode 161 poled to allow positivepotential only to appear at the point P2. Said diode serves to protectthe transistor 162 against overload. The transistor 162 is combined witha second transistor 163 to form a differential amplifier device. Thecontrol voltage produced by the operational amplifier 121 (FIG. 5) issupplied to the base of the transistor 163 through the conductor 126 anda resistor 164. The transistors 162, 163 have a common emitter resistor165. The collector of the transistor 163 is connected to the conductor127 carrying the positive operating voltage through a resistor 166 andis also directly connected to the base of the transistor 167 the emitterof which is connected to the conductor 127. The transistor 167constitutes the first stage of a pulse amplifier the second and thirdstages of which are constituted by the transistors 168 and 169,respectively. A fraction of the negative operating voltage is suppliedby a potentiometer arrangement 172, 173 to the collector of thetransistor 169 through resistors 171, 170. The conductor 82 connected tothe firing lead of one of the thyristors 69 is connected to thecollector of the transistor 169 through the series resistors 174 and170, a capacitor 175 being connected in parallel with the resistor 170.

In FIG. 8, the diagram a represents the voltage pulses supplied by theconductor 133, the diagram b represents the potential of the point P3,the diagram c represents the potential of the point P2 (the full line176) and the control voltage supplied by the amplifier 121 through theconductor 126 (the dotted line 177), the diagram d represents thepotential of the junction point P4 of the resistors 170, 171, and thediagram e represents the triggering pulses transmitted through theconductor 82.

The voltage pulse transmitted by the conductor 133 (FIG. 8a) renders thetransistor 153 conducting for the duration of said pulse, whereby thepotential of the point P3 is rendered negative (FIG. 8b). A chargingcurrent than flows from the positive operating voltage conductor 93through conductor 127, resistor 157, capacitor 154, resistors 156, 155,the transistor 153 and the conductor 128 to the negative operatingvoltage conductor 94. The charging of the capacitor 154 produces a dropof the potential of the point P2 (line 176 in FIG. 8c). The suddenvoltage drop r at the beginning of the charging period is caused by thepotentiometer action of the resistors 155-156 and 157. The charging ofthe capacitor goes on until the potential of the point P2 has dropped tothe zero level. The potential then remains constant (the portion s ofthe diagram). Termination of the voltage pulse transmitted through theconductor 133 causes the transistor 153 to be stored to thenon-conducting state. The capacitor 154 is then discharged through theresistor 158 in series with the diodes 159, 160. The resistance of thedischarge resistor 158 is much smaller than the total resistance of thecharging resistors 155, 156, 157. The discharge therefore proceedsquickly (the part μ of the diagram). The point P2 as well as the pointP3 are now again at the potential of the positive operating voltageconductor (the part v of the diagram).

The transistor 162 is conducting as long as the emitter potentialexceeds the emitter potential of the transistor 163, said last-mentionedpotential is equal to the control voltage of the conductor 126. As soonas said last-mentioned emitter potential is the larger one, thetransistor 163 is rendered conducting and blocks the transistor 162.This occurs at the intersection of the curve 176 and line 177 in FIG.8c. The resulting pulse is amplified by the three-stage pulse amplifier.When the transistor 169 of the last amplifier stage is renderedconductive, a positive potential is imparted to the point P4 (FIG. 8d).This potential leap causes a current surge through the capacitor 175 anda corresponding pulse through the conductor 82 (FIG. 8e). The triggeringcurrent is cut off as soon as the discharge of the capacitor 154 causesthe potential at the point P2 to rise to a positive value sufficient tomake the emitter potential of the transistor 162 exceed the one of thetransistor 163, resulting in restoring the transistor 163 to itsnon-conducting state. This occurs at the intersection of the chargingline μ (FIG. 8c) and the line 177. The transistor 169 as well isrendered non-conducting, causing the potential at the point P4 to dropimmediately to a negative value (FIG. 8d) determined by thepotentiometer 172, 173. The negative bias thus produced preventsspurious firing of the thyristor. No refiring of the thyristor can occuruntil the next voltage pulse (FIG. 8a) has been supplied by theconductor 133.

The described triggering circuit has the advantage that the triggeringangle does not vary with the manufacturing tolerances of thetransistors, which may be considerable. The capacity of the capacitor154 and the resistance of the resistors 155, 156, 157 are the onlyfactors governing the charging of the capacitor 154. Said elementscannot be assumed to be absolutely alike in all of the three triggeringunits. Adjustment of one element only in each of the triggering units,to wit, the resistor 155, is however, sufficient to provide identicaltime constants and consequently identical charging diagrams in all ofthe three triggering units. The steepness of the charging line 176 issufficient to provide a sharply defined intersection with the horizontalline 177 representing the control signal sent out by the operationalamplifier 121.

In a commercial form of a welding power supply of the type representedin FIG. 5 having a current range of 30 to 400 amperes, the weldingcurrent feed-back factor amounts to about 3 volts/ampere. The change ofthe arc current caused by a line voltage variation of ± 10% is below 1%for current settings above 100 amperes. At current settings below 100amperes, the arc current change for 10% line voltage variation graduallyincreases towards a maximum of about 2.5 percent for the lowest currentsetting (30 amps.).

I claim:
 1. A direct current power supply for manual arc welding havinga steeply drooping, adjustable volt-ampere characteristic comprisingapolyphase transformer having a plurality of secondary output leadscarrying mutually phase displaced voltages, a polyphase rectifier systemconnecting said secondary output leads to a pair of D.C. welding currentconductors, said rectifier system comprising a set of triggeredrectifier valves connected one in series with each of said secondaryoutput leads, said triggered rectifier valves having each a triggerterminal for initiating current flow through the valve, means forperiodically supplying igniting pulses to each of said triggerterminals, including means for causing the angular position of saidpulses to vary in response to a direct current control signal, and,means for producing said direct current control signal comprising thefollowing means, to wit, a. means for producing an adjustable directcurrent reference signal, b. means including a welding current sensingmeans for producing a welding current feed-back direct current signalbearing a substantially fixed proportion to the welding current carriedby the pair of welding current conductors, c. means including a weldingvoltage sensing means for producing a welding voltage feed-back directcurrent signal proportional to the welding voltage between said weldingcurrent conductors, d. summing means for combining said referencesignal, said welding current feed-back signal and said welding voltagefeed-back signal into a compound signal constituting the control signalaforementioned, both of said feed-back signals being supplied to saidsumming means in opposing relationship to said reference signal, saidmeans for producing a welding voltage feed-back signal including meansfor adjustment of the slope of the volt-ampere characteristic of thepower supply through adjustment of the ratio of said welding voltagefeed-back signal to the welding voltage independently of the ratio ofthe welding current feed-back signal to the welding current.
 2. A directpower supply having a steeply drooping volt-ampere characteristic formanual arc welding comprisinga polyphase transformer having a pluralityof secondary output leads carrying mutually phase displaced voltages, apolyphase rectifier system connecting said secondary output leads to apair of D. C. welding current conductors, said rectifier systemcomprising a set of triggered rectifier valves connected one in serieswith each of said secondary output leads, said triggered rectifiervalves having each a trigger terminal for initiating current flowthrough the valve, means for periodically supplying igniting pulses toeach of said trigger terminals, including means for causing the angularposition of said pulses to vary in response to a direct current controlsignal, and, means for producing said direct current control signalcomprising the following means, to wit, a. means for producing anadjustable direct current reference signal, b. means including a weldingcurrent sensing means for producing a welding current feed-back directcurrent signal bearing a substantially fixed proportion to the weldingcurrent carried by the pair of welding current conductors, c. meansincluding a welding voltage sensing means for producing a weldingvoltage feed-back direct current signal proportional to the weldingvoltage between said pair of welding current conductors, d. summingmeans for combining said reference signal, said welding currentfeed-back signal and said welding voltage feed-back signal into acompound signal constituting the control signal aforementioned, both ofsaid feed-back signals acting in opposition to said reference signal insaid summing means, said means for producing a welding current feed-backsignal, said summing means, said means for controlling the firing angleof the triggered rectifier valves and said rectifier system constitutinga welding current feed-back loop arranged to provide a welding currentfeed-back factor not less than 1.5 volts per ampere, and said means forproducing a welding voltage feed-back signal including means foradjustment of the ratio of said welding voltage feed-back signal to thewelding voltage independently of the ratio of the welding currentfeed-back signal to the welding current to provide a selected, steepslope of the volt-ampere characteristic of the power supply.
 3. A directcurrent welding power supply as claimed in claim 2 in which said weldingcurrent feed-back loop provides a welding current feed-back factor notless than 2.5 volts per ampere.